This invention relates generally to flow measurements for process fluid control. Specifically, the invention concerns compensation techniques for magnetic flowmeters subject to upstream or downstream flow pipe disturbances.
Precise and accurate flow control is critical to a wide range of fluid processing applications, including bulk fluid handling, food and beverage preparation, chemistry and pharmaceuticals, water and air distribution, hydrocarbon extraction and processing, environmental control, and a range of manufacturing techniques utilizing thermoplastics, thin films, glues, resins and other fluid materials.
The flow measurement technologies used in each particular application depend upon the fluids involved, and on the relevant process pressures, temperatures and flow rates. Representative technologies include turbine devices that measure flow as a function of mechanical rotation, Pitot sensors and differential pressure devices that measure flow as a function of the Bernoulli effect or pressure drop across a flow restriction, vortex and Coriolis devices that measure flow as a function of vibrational effects, and mass flowmeters that measure flow as a function of thermal conductivity.
Magnetic flowmeters or “mags” distinguish from these technologies in that they characterize flow via Faraday's Law, which depends upon electromagnetic interactions rather than mechanical or thermodynamic effects. In particular, magnetic flowmeters rely upon the conductivity of the process fluid, and the electromotive force or EMF induced as the fluid flows through a region of magnetic field.
Magnetic flowmeters provide particular advantages in “dirty” (erosive and corrosive) flow environments, including hydraulic fracturing and hazardous chemical processing applications, and when other techniques require an inappropriate pressure drop or flow restriction. Because magnetic flowmeters depend upon electromagnetic induction, however, they also pose a number of engineering challenges. In particular, magnetic flowmeters are “single point” measuring devices, rather than multi-point (averaging) devices, making them susceptible to swirl and other non-uniform disturbances in the process flow.
In order to reduce the effect of flow pipe disturbances, installation guidelines typically require a “straight-pipe” installation zone, extending both upstream and downstream of the magnetic flowmeter. Unfortunately, physical, financial and time constraints sometimes make it impossible to install the flowmeter strictly according to these recommendations. There is thus a need for improved magnetic flow measurement techniques that compensate for flow pipe disturbances, improving precision and accuracy while reducing installation cost and providing greater flexibility in system design options.